1. Field of the Invention
This invention relates to communications, and, more particularly, to a system and method for reducing distortion using predistortion.
2. Description of Related Art
An ideal power amplifier amplifies an input signal with no waveshape alteration. The ideal power amplifier is therefore characterized as having a transfer function (input signal vs. output signal) which is linear with no transfer function discontinuities. In practice, however, a power amplifier has a transfer function with nonlinear and “linear” regions. Whether the power amplifier is operating in a linear or nonlinear region depends in part on the amplitude of the input signal. For the power amplifier to achieve as near to linear operation as possible, the power amplifier is designed to operate within its linear region given the range of possible input signal amplitudes. If the input signal has an amplitude which causes the power amplifier to operate outside the linear region, the power amplifier introduces nonlinear components or distortion to the signal. When the input signal possesses peak amplitudes which cause the amplifier to compress, to saturate (no appreciable increase in output amplitude with an increase in input amplitude) or to shut-off (no appreciable decrease in output amplitude with a decrease in input amplitude), the output signal is clipped or distorted in a nonlinear fashion. Generally, an amplifier is characterized as having a clipping threshold, and input signals having amplitudes beyond the clipping threshold are clipped at the amplifier output. In addition to distorting the signal, the clipping or nonlinear distortion of the input signal, generates spectral regrowth or adjacent channel power (ACP) that can interfere with an adjacent frequency.
In wireless communications systems, high power amplification of signals for transmission are commonly encountered with very large peak to average power ratios (PAR). For example, in a time division multiple access (TDMA) system, such as Global System for Mobile Communications (GSM) or North American TDMA, when multiple carrier signals are combined for amplification with a power amplifier, the resulting PAR is about 9-10 dB for a large number of carriers. In a code division multiple access (CDMA) system, a single loaded 1.25 Mhz wide carrier can typically have a PAR of 11.3 dB. For orthogonal frequency division multiplexing (OFDM), multicarrier signals can have a PAR of up to 20 dB. These signals have to be amplified fairly linearly to avoid generating ACP.
Unfortunately, efficiency of the base station amplifier is inversely related to its linearity. To achieve a high degree of linearity, the amplifiers are biased to operate in the class A or “slight” class AB (meaning class AB operation that is closer to class A than to class B). Maximum AC to DC efficiency achievable for class A operation is 50%, whereas that of a class AB amplifier is between 50 and 78.5% (the latter representing the maximum efficiency of a class B amplifier). The closer the particular class AB operation is to class A, the lower the maximum efficiency.
Typically, strict linearity requirements in modern wireless communication systems dictate the use of the relatively inefficient class A or slight class AB modes. As a result, significant DC power is dissipated by the amplifiers, thereby generating heat which must be controlled to avoid degrading amplifier performance and reliability. Hence, the use of elaborate heat sinks and fans become a necessary by-product of the high linearity system. Naturally, these measures add to the cost, size and weight of the base station equipment. As the number of wireless communications users continues to grow, so do the number of base stations and the need to keep them small, light and inexpensive. Thus, a great deal of research has focused on the quest to improve amplifier efficiency in these and other systems.
Various linearization methods are used to enable the use of more cost-effective and more power efficient amplifiers while maintaining an acceptable level of linearity. Feed-forward correction is routinely deployed in modern amplifiers to improve the linearity of the main amplifier with various input patterns. The essence of the feed-forward correction is to isolate the distortion generated by the main amplifier on a feed forward path. The distortion is provided to a correction amplifier on the feed forward path which amplifies the distortion. The distortion on the feed forward path is combined with the distortion on the main signal path to cancel the distortion on the main signal path. Predistortion techniques distort the input signal prior to amplification by taking into account the transfer function characteristics for the amplifier. As such, the desired amplified signal is achieved from the predistorted input signal by intentionally distorting the signal before the amplifier, so the non-linearity of the amplifier can be compensated.
FIG. 1 shows a general functional block diagram of an adaptive power amplifier predistortion system 10. The baseband digital input signal un on a main signal path 12 is input into the predistortion function 14 (A(.)) to produce a predistorted output xn where n is the time index. After digital to analog conversion by digital to analog (D/A) converter 16, the resulting analog signal is frequency up-converted in an up-conversion process 18 to radio frequency (RF). The analog RF signals are amplified by power amplifier 20 for transmission over the air using antenna 22. A replica of the amplified analog RF signals is coupled off the main signal path 12 onto a predistortion feedback path 24. The amplified analog RF signals on the predistortion feedback path 24 are down-converted by a down-conversion process 26.
The down-converted analog signals on the predistortion feedback path 24 are provided to an analog to digital (A/D) converter 28 for conversion into the digital domain. The resulting digital signal, which represents the output of the amplifier 20, is provided to an amplifier characteristics estimation block 30 along with the digital baseband signal xn which represents the corresponding input to the amplifier 20. Given the digital signals xn prior to amplification and the digital signals yn resulting from the amplification of the analog and frequency converted versions of the digital signals xn, the amplifier characteristics estimation block 30 can determine the characteristics or model function of the amplifier 20. Once the model or characteristics function of the amplifier 20 is estimated, a predistortion calculation process 34 determines the predistortion function as the inverse of the amplifier characteristics function, and the predistortion function 14 (A(.)) applied to the input signal un is updated based on the predistortion calculation process 34.
FIG. 2 is a general block diagram for an adaptive digital predistortion system. An amplifier 40 is characterized by a baseband function B(.) with complex inputs and complex outputs. There are many methods for adaptive digital predistortion which are generally divided into two steps as mentioned above. First, an amplifier characteristics estimation block 42 determines the characteristics or model function B(.) of the amplifier 20, where proper modeling and parameter estimation based the model function is needed. Using input samples xn and corresponding amplified output samples yn, the amplifier characterization estimation block 34 adapts the model for the amplifier 40 over time. Second, the predistortion calculation process 44 determines the predistortion function as the inverse of the model function B(.) and updates the predistortion function 46 applied to the digital input signal un.
In general, the output yn of the amplifier 40 is a function of input samples {xn,xn−1,xn−2 . . . } and previous output samples {yn−1, yn−2, . . . }. Let b be the vector of coefficients for B(.), then the estimation of the amplifier characteristics is obtaining b from the following equation:b=arg min E [|B(xn,xn−1,xn−2, . . . yn−1,yn−2, . . . )−yn|2],where E[.] means expected value and arg min f(.) means the arguments of the function f(.) that makes f(.) minimum. In other words, b is the vector of coefficients that minimizes the power of the estimation error, B(.)−yn. The predistortion function A(.) is produced by determining the inverse function of B(.).
Due to the potential for high peak powers in wireless communications signals, CDMA, TDMA and frequency division multiple access (FDMA) base stations typically use radio frequency (RF) amplifiers operating in class AB mode and biased with a high current to be able to handle those peak powers. The efficiency of these amplifiers is typically less than 10%. This low efficiency leads to higher power consumption, lower overall reliability and higher operating temperatures. Accordingly, there is a need for a more efficient power amplifier architecture which can amplify signals having potentially high peak powers in a linear fashion.